Field of the Invention
The invention relates to a method for reducing cationic impurities in the cooling-water loop of a light water reactor, and to the application of the method, the cooling-water loop containing a cationic solution. Furthermore, the invention relates to a method and the application of the method for dosing lithium in a cooling-water loop, containing cationic impurities, of a light water reactor. The invention also relates to a device having an electrodialysis unit that is connected on one side to a water loop, and connected on the other side to a concentrate loop. Finally, the invention relates to a cooling-water system of a light water reactor. In nuclear power plants having a light water reactor, boron in the form of boric acid (H3BO3) is added to the reactor coolant (water) for the purpose of absorbing neutrons. It serves to protect the reactor components against radiation and is practiced both in the case of boiling water reactors and, in particular, also in the case of pressurized water reactors. The boric acid has a side effect in that it lowers the pH of the cooling water, an effect that is not desired for reasons of protecting the components against corrosion, and it must be at least partially compensated for by adding an alkalizing agent. Isotopically pure lithium-7 (7Li) is normally used as alkalizing agent because it, on one hand, has virtually no undesired nuclear reactions with the neutrons present in the reactor core and, on the other hand, is continuously formed itself in the reactor by the nuclear reaction 10B(n,xcex1)7Li proceeding during the neutron absorption by the active boron isotope 10B.
The isotopic purity of the lithium used is necessary because the other isotope, 6Li, present in the natural composition of the lithium has a very strong reaction with the neutrons, which produces tritium as reaction product. The excessive enrichment of tritium in the cooling water is undesired. The 7Li is added in the form of lithium hydroxide solution (LiOH) and is present as a monovalent cation 7Li+ as a consequence of the dissociation of LiOH. It is expensive to produce isotopically pure 7Li. Isotopically pure 7Li is, therefore, very valuable and it is desirable to handle it economically.
The continuous reformation of 7Li as a consequence of the nuclear reaction of the 10B, and the only slight losses due to cooling water leaks cause an increase in the 7Li concentration in the cooling water in the course of a fuel cycle of a light water reactor. This relates, in particular, to cooling water in the primary loop of a light water reactor, in particular, cooling water in the primary loop of a pressurized water reactor. The 7Li concentration in the cooling water increases, in particular, at the start, that is to say, in an early time domain of a fuel cycle. Because, upon overshooting a concentration of approximately 2 ppm, lithium can cause corrosion on reactor components, it is necessary to withdraw a sufficient quantity of 7Li+ again from the cooling water. The result is chiefly to reduce corrosion of fuel rod cladding tubes that enclose the nuclear fuel in the fuel rod.
Because, however, the density of fissile material in the fuel also reduces in the course of a fuel cycle, it is also necessary to reduce the concentration of neutron-absorbing boron in the cooling water in the course of a fuel cycle. The reduction is usually achieved by extracting the boron-containing cooling water from the cooling-water loop and the feeding in of an equally large quantity of boron-free water. In such a process, lithium is also removed from the loop with the boron-containing cooling water, and is not supplemented by the feeding in of the normally completely demineralized water. As a result, therefore, the reduction in boron concentration also lowers the lithium concentration. At the end of a fuel cycle, the cooling-water exchange masses are substantially enlarged, in order to achieve an adequate lowering of the boron concentration. Normally, LiOH solution is then fed into the cooling-water loop to maintain a required lithium concentration, in particular, into the primary loop of a pressurized water reactor.
It is, therefore, necessary, depending on the operating cycle of a light water reactor, to dose the content of lithium in the cooling water of the light water reactor. The dosing feeds lithium to the cooling water, in particular, chiefly at early times in the fuel cycle, and withdraws it from the cooling water, in particular, at later times in a fuel cycle.
Because radioactive materials are continuously produced by the nuclear fission in the reactor and by the activation of material as a consequence of the neutron emission, it is unavoidable that the materials pass partly into the cooling water and contaminate the cooling water. These materials can be present in the cooling water in a different chemical form and be partially undissolved and partially dissolved as anions or cations. This relates, in particular, to emitting nuclides, chiefly cesium and cobalt, which are present as cations. Because the separation of lithium from the cooling water is normally performed by employing the positive electric charge of the lithium cation, a portion of the cationic, radioactive impurities is also separated from the cooling water together with the lithium. The valuable, isotopically pure, separated lithium is, thereby, contaminated and can, therefore, not be reused, as a rule.
For example, to lower the lithium concentration, the cooling water is normally led through ion exchangers that include cation exchanger resins. These cation exchanger resins bind the lithium ions contained in the water flowing through virtually completely to the resin and simultaneously output an equivalent quantity of hydrogen ions to the water. However, they also bind the cationic impurities, and, therefore, concentrate emitting nuclides. If they are saturated and finally ineffective for the lithium withdrawal, they are replaced by new resins. A regeneration of the exchanger resins, in the case of which the very expensive 7Li could be recovered and, if required, fed into the cooling-water loop again, has already foundered on the fact that, in such a case, the concentrated impurities are also released together with the 7Li. The depleted exchanger resins are, therefore, to be disposed of as highly emissive special waste.
The invention proceeds from the fact that the ion concentrations in two solutions can be set if an electrodialysis is performed between the loops of the two solutions. In this case, electrodialysis means ion transport through a membrane configuration having at least one membrane separating the loops, it being possible to control the direction and throughput of the ion transport by applying an electric voltage. Examples of such electrodialysis methods are described, for example, in German patent applications 19747077.7 and 19747076.9. However, the method described in German patent application 19747077.7, in particular, has the disadvantage that there is a lowering of the boron concentration at the same time as a lowering of the lithium concentration.
It is accordingly an object of the invention to provide a method and device for reducing cationic impurities and for dosing lithium in the cooling water of a light water reactor, and a cooling-water system of a light water reactor having such a device that overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices and methods of this general type and that improves methods and devices that operate using such electrodialysis.
With the foregoing and other objects in view, there is provided, in accordance with the invention, a method for reducing cationic impurities in a cooling water including the steps of providing a light water reactor having a cooling-water loop containing a cationic solution, a concentrate loop containing a medium in which a heightened cation concentration is produced, and a selective ion exchanger disposed in the concentrate loop, guiding cooling water of the cooling-water loop through a first side of an electrodialysis unit, guiding the medium of the concentrate loop through a second side of the electrodialysis unit, and filtering out cationic impurities from the medium in the selective ion exchanger.
With the objects of the invention in view, there is also provided a method for dosing lithium in cooling water including the steps of providing a light water reactor having a cooling-water loop containing a cooling water with cationic impurities, a concentrate loop containing a medium having a heightened cation concentration, and a selective ion exchanger disposed in the concentrate loop, guiding the cooling water of the cooling-water loop through a first side of an electrodialysis unit, guiding the medium of the concentrate loop through a second side of the electrodialysis unit, applying an electric voltage in the electrodialysis unit to control an exchange of lithium cations between the cooling water in the cooling-water loop and the medium in the concentrate loop, and filtering out cationic impurities from the medium in the selective ion exchanger.
The invention proceeds from the prior art methods in a method for dosing lithium in a cooling water, containing cationic impurities, in a cooling-water loop of a light water reactor. In such a case, the cooling water is guided through a first side of an electrodialysis unit, and a medium is guided in a concentrate loop through the second side of the electrodialysis unit. A heightened cation concentration is present in the medium. The application of an electric voltage to the electrodialysis unit controls the exchange of lithium cations between the cooling water in the cooling-water loop and the medium in the concentrate loop.
According to the invention, the cationic impurities are filtered out by a selective ion exchanger. A cation exchanger, in particular, is suitable for such a purpose. The selective ion exchanger is disposed in the concentrate loop; the cleaning, therefore, takes place in a selective ion exchanger that is not flowed through by the cooling water but by the medium with the concentrated cations.
The invention proceeds from the surprising finding that the decontamination factor of a selective ion exchanger is a function of the concentration of the solution led through the ion exchanger. The decontamination factor specifies the ratio of the activity upstream of the ion exchanger to the activity downstream of the ion exchanger. It is substantially lower in a solution of low ion concentration than in the case of a higher concentration. Moreover, it can depend on the pH value and conductivity of the solution. Consequently, a membrane configuration exchanging selectively for cations, or a selective ion exchanger, is only conditionally effective as a membrane of the electrodialysis unit with the aid of which, for example, the Li concentrations in two loops are set. However, substantially better use can be made of the selectivity of an ion exchanger if it is used only as a filter for the ions to be selected in the correct loop.
It is preferred to employ a cation exchanger used as a dialysis membrane when utilizing an electrodialysis unit for dosing lithium in a cooling water containing cationic impurities. Such a configuration need be replaced not at all, or only at lengthy intervals. Rather, it is only the ion exchanger disposed in the concentrate loop that becomes depleted, in which case a substantially larger quantity of cationic impurities is then exchanged against H+ ions, as compared to a situation where the exchanger were disposed in the water loop with the lower concentration. Consequently, relatively low quantities are already sufficient in the selective ion exchanger to effectively lower the concentration of cationic impurities in the system (that is to say, in the water loop and in the concentration loop). Moreover, the ratio of the impurities to the cations that are required for the water loop is shifted strongly to the benefit of the impurities in the depleted exchanger. As a result, there are, therefore, only low losses of 7Li, for example, in the reactor and small quantities of ion exchanger requiring disposal.
In accordance with another mode of the invention, it is preferred that the medium in the concentrate loop is guided through a partial loop with an accumulator for concentrating the cations and through a further partial loop, likewise connected to the accumulator, with the selective ion exchanger for filtering out the cationic impurities.
In accordance with an added mode of the invention, the medium guided in the concentrate loop is stored and fed to the cooling water when required. Such a configuration has the advantage that there is no need at a later instant in the fuel cycle to add to the cooling water additional isotopically pure lithium that is to be brought in. As such, according to the method, the isotopically pure lithium withdrawn at an earlier instant can be used again later in the cooling-water loop because it is effectively freed from impurities.
In accordance with a further mode of the invention, the two sides of the electrodialysis unit (that is to say, the cooling-water loop and the concentrate loop) are preferably separated by a membrane configuration that is virtually permeable to cations but largely prevents the passage of anions (cation exchanger membrane). As a result, there is virtually no transport of boron anions between the cooling water and the medium during transport of lithium (cations). Specifically, it is advantageously the aim to virtually suppress the transport of anions, in particular, boron-containing anions, through the electrodialysis unit. As a is result, the Li-containing cooling water is dosed and/or cleaned without changing the boron concentration. Thus, boron and lithium can be dosed independently of one another in each case when required.
In accordance with an additional mode of the invention, the cooling water of the cooling-water loop is optionally guided through the first side of the electrodialysis unit and, correspondingly, the medium of the concentrate loop is guided through the second side of the electrodialysis unit. It is also optionally possible to guide the cooling water through the second side of the electrodialysis unit, and the medium through the first side. Accordingly, it is possible to reverse the polarity of the voltage at the electrodialysis unit during the method. The reversal has the advantage that the electrodes of the electrodialysis unit are cleaned of deposits upon exchanging the loops for water and medium and simultaneously reversing the polarity of electric voltage.
In accordance with yet another mode of the invention, it is advantageous, furthermore, to extract cooling water from the cooling-water loop and store the water. The accumulator is preferably vented such that, in particular, H2O2 gas mixtures are withdrawn. In particular, the concentrate loop and/or the cooling-water loop are/is vented such that here, as well, H2O2 gas mixtures, in particular, are withdrawn.
In accordance with yet a further mode of the invention, it is also advantageous that the cooling-water loop is guided through an H2O2 recombiner.
In accordance with yet an added mode of the invention, the method is executed such that cations are transported from the cooling-water loop into the concentrate loop through a cation exchanger membrane. Oxygen is produced in this case in the cooling-water loop on the anode side of the electrodialysis unit, that is to say, the first side of the electrodialysis unit. It is, therefore, advantageous that at least one portion of the cooling water is fed to a cooling water sectional line and admixed to at least this part of the cooling water H2, and at least this portion of the cooling water is guided through an H2O2 recombiner. Such a process has the advantage that cooling water is freed of oxygen and corrosive damage to reactor components is thereby reduced.
In accordance with yet an additional mode of the invention, the method is executed such that cations are transferred from the concentrate loop into the cooling-water loop through a cation exchanger membrane. The cations are preferably lithium cations. That is to say, according to the development the method according to the invention is executed virtually to feed lithium back into the cooling-water loop. In particular, for such a purpose, the cooling water is fed to a bypass line that bypasses an H2O2 recombiner. The point is that, in the case of the development of the method just named, no additional oxygen is produced in the cooling-water loop on the first (anode) side of the electrodialysis unit by the electrodialysis process, and so the previously named H2O2 recombiner is now bypassed by the bypass line.
The above-named method can, in particular, be used not only to dose lithium, but chiefly applied also to reduce cationic impurities in the coolant.
In accordance with again another mode of the invention, the method reduces cationic impurities in a cooling-water loop, containing a cationic solution, of a light water reactor. In such a case, cooling water from the cooling-water loop is guided through a first side of an electrodialysis unit, and a medium of a concentrate loop is guided through a second side of the electrodialysis unit. In the case of the method according to the invention, a heightened cation concentration is produced in the medium, the cationic impurities being filtered out of the medium in a selective ion exchanger in the concentrate loop.
In accordance with again a further mode of the invention, the method is, however, also applied for dosing lithium in the coolant. In particular, it is advantageous also to execute the last-named method using one of the above-named developments.
With the objects of the invention in view, in a light water reactor having a cooling-water loop containing a cooling water with cationic impurities and a concentrate loop containing a medium having a heightened cation concentration, there is also provided a device for at least one of reducing cationic impurities and dosing lithium in the cooling water, the device including an electrodialysis unit having two sides, one of the sides connected to the cooling-water loop and another of the sides connected to the concentrate loop, and a selective ion exchanger disposed in the concentrate loop for at least one of reducing cationic impurities and dosing lithium in the cooling water.
The device according to the invention is suitable, in particular, for carrying out one of the above-named variants of methods, or developments of one of the variants. According to the prior art, such a device has an electrodialysis unit that is connected on a first side to a cooling-water loop, and on another, second side to a concentrate loop. According to the invention, a selective ion exchanger is connected in the concentrate loop in the case of the device. In particular, the sides of the electrodialysis unit are separated by a membrane configuration, exchanging only cations, having at least one cation exchanger membrane. The membrane configuration is advantageously virtually permeable to lithium cations. On the other hand, it is advantageous that the membrane configuration is virtually impermeable to boron anions.
In accordance with an added feature of the invention, the electrodialysis unit has a device controlling an exchange of lithium cations between the cooling water in the cooling-water loop and the medium in the concentrate loop by applying an electric voltage.
In accordance with an additional feature of the invention, the two sides are separated by a membrane configuration having at least one cation exchanger membrane exchanging substantially only cations.
The ion exchanger is selective, in particular, for cationic cesium and/or cobalt nuclides. The ion exchanger is advantageously configured for such a purpose as a cation exchanger. In particular, it contains phenol- and/or formaldehyde-based resins, specifically, those that are referred to under the commercial names of Duolite and/or Amberlite. These have the advantage that they are selective, in particular, for cationic cesium and/or cobalt nuclides.
In accordance with yet another feature of the invention, an H2O2 recombiner is connected to the water loop. The recombiner serves to recombine the oxygen produced, in particular, in the electrodialysis unit. The H2O2 recombiner advantageously includes, for such a purpose, a catalyst bed filled with an anion exchanger. The anion exchanger preferably includes a palladium-doped resin. Particularly suitable for such purposes is a palladium-doped resin that is available under the commercial name of Lewatit and is described, for example, in more detail in the company publications of Bayer AG.
In accordance with yet a further feature of the invention, the electrodialysis unit is connected to the water loop and the concentrate loop through a switching valve. The water loop can be connected optionally through the switching valve to the first or else second side of the electrodialysis unit. Accordingly, the connection of the concentrate loop can then be switched over to the electrodialysis unit. That is to say, either the water loop can be connected to the first side, and the concentrate loop can be connected to the second side of the electrodialysis unit, or, instead, the water loop can be connected to the second side, and the concentrate loop can be connected to the first side. The connections of the loops can, therefore, be switched over through the switching valve. The electric voltage present at the electrodialysis unit can be switched over according to a routing of the water loops. The configuration has the advantage that an electrode of the electrodialysis unit can optionally be used as cathode or else as anode, and so the direction of the electrodialysis process can be reversed. Consequently, an electrode of the electrodialysis unit can be freed from deposits that accumulate once a switching direction has been selected.
In accordance with yet an added feature of the invention, an accumulator having a sealable feed-in opening into the water loop is connected to the concentrate loop. The accumulator in the concentrate loop advantageously serves to store the lithium-hydroxide solution, and the feed-in opening is to be opened, when required, to feed back the lithium-hydroxide solution, such that isotopically pure 7Li is fed back into the water loop.
In accordance with yet an additional feature of the invention, the cooling-water system of a light water reactor includes a coolant cleanup plant, a coolant storage device, a coolant evaporator plant, and a device according to one of the developments of the invention. In such a case, the water loop of the device according to the invention is connected to the coolant cleanup plant or the coolant storage device or the coolant evaporator plant.
With the objects of the invention in view, there is also provided a light water reactor including a cooling-water loop containing a cooling water with cationic impurities, a concentrate loop containing a medium having a heightened cation concentration, an electrodialysis unit having two sides, one of the sides connected to the cooling-water loop and another of the sides connected to the concentrate loop, and a selective ion exchanger disposed in the concentrate loop for at least one of reducing cationic impurities and dosing lithium in the cooling water.
Other features that are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method and device for reducing cationic impurities and for dosing lithium in the cooling water of a light water reactor, and a cooling-water system of a light water reactor having such a device, it is, nevertheless, not intended to be limited to the details shown because various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.